Given that there are U.S crew that fly transcon turns, just not common, here are other points to consider:
(1) Many of those that allow these turns are flying for low cost carrier. LCC have shorter and tigher turnaround time between flights. They also fly narrow body aircraft which allows for quick unload/reload. Given that in QF's situation the crew will stay on the same aircraft, it certainly takes a lot longer to deplane, unload bags, load new bags and board a 747 than a 737 or an A320 that many of the LCC fly. This adds to their duty time.
(2) Catering adds more time. Because they only serve snacks and beverages, catering on LCC can be boarded from the originating station but be sufficient for the whole transcon turn. On legacy carriers and probably QF, the inflight service will probably comprise close to a full international meal service judging by the flight time. Not to mention its harder work for these crew since they are not simply handing out snacks, in my opinion.
(3) Remember that airlines "knowingly" violate union work rules all the time. Just because somebody said they have done a transcon turn does not mean they were suppose to be assigned such a trip. Often, such trips are assigned last minute while on reserve. They could be barely legal only by minutes but have not choice but to fly. Or they could go illegal while in the air at which time there is nothing you can do and the company simply shrugs its shoulders. This is particularly a favorite for airlines if they know you have a days off following that trip.
I am not necessarily in support or against the proposed change, but rather feel that it needs to be a win win situation. If they want a transcon turn after an international trip, than pad both ends with longer layover. If not, cut the layover to bare minimum but break up the trip into two segments. They could potentially run into problems where the transcon crew go illegal on route, have their layover extended back at LAX (or not??) which means they can't work the scheduled flight home to Oz. Then QF will probably have to deadhead an entire crew to LAX to work the flight home or cancel the flight. Not sure how this will play out.
Just my two cents.